Comment by lefra

Some countries solve that by just banning mail-in voting and moving paper ballots.

In France for example, voting happens on a specific day with transparent ballot boxes, with the counting happening in the polling station (you may stay for the whole day in your polling station if you want to oversee the whole process). Yes this is problematic for voters in foreign countries, because they need to go to the embassy to vote, but the voting protocols chose transparency over practicality.

IIRC, deployed military get a polling station in their military base. A voter may also designate someone in the same polling station as theirs to vote in their place.